Investigations in rodents have convincingly shown that increases in non-shivering thermogenesis (NTS) are extremely effective in reducing obesity. However, we know very little about alternative mechanisms of thermogenesis that regulate body weight in either animal models or in humans. Although children have active NTS, it is not thought to contribute significantly to thermogenesis in the adult human. A major obstacle to finding alternative thermogenic mechanisms has been the dominant and pervasive effects of NTS in rodent models. Because of its high expression NTS prevents the independent analysis of alternative thermogenic mechanisms. This proposal seeks to identify alternative thermogenic mechanisms by making use of a genetic model created in mice in which the uncoupling protein (UCP1), which is essential for NTS, has been inactivated. Contrary to expectations these mice show increased resistance to dietary obesity, suggesting that alternative mechanisms of thermogenesis consume more calories to regulate body temperature than does NTS. This proposal will take advantage of characteristics of these mice to guide experiments to identify UCP1-independent thermogenic mechanisms. Specific Aim 1 will use microarray analysis of gene expression and proteomics of mitochondrial proteins to find genes with altered expression during the slow adaptation of UCP1 deficient mice to the cold. Specific Aim 2 will combine UCP1 deficiency with mutants causing leptin, catecholamine and fatty acid oxidation deficiency to determine whether these systems are involved in alternative mechanisms of thermogenesis by measuring adiposity, energy expenditure and changes in the expression of genes known to reflect the state of fatty acid oxidation. Specific Aim 3 seeks to map genetic loci that cause thermogenic heterosis in hybrid mice with the goal of identifying alternative thermogenic genes. This aim depends on the high throughput mapping strategies with single nucleotide polymorphic markers in intercross progeny that are deficient in UCP1. Thus, three independent experimental approaches are focused on identifying thermogenic genes that could have a significant impact on the development of obesity in both children and adults. Concurrent with gene identification, the experiments will provide phenotypic information on how these genes affect the development of excessive adiposity.